TILBegin-EndExtensionUsingTXL

A TXL solution to TIL Chairmarks #1.3, the begin-end syntax extension for the Tiny Imperative Language.

TXL is designed for implementing language extensions, so adding a new syntactic feature is simple in TXL using Grammar Overrides. The "..." in the "redefine" is not a shorthand in this example, rather a language feature of TXL that denotes an extension to the existing nonterminal alternatives.

Since there is no semantics for begin-end in TIL, we don't add any transform for this yet.

-- JamesCordy - 10 Oct 2005

File "TILbeginparser.Txl"

% TXL parser for begin-end language extenssion to Tiny Imperative Language
% Jim Cordy, October 2005

% Begin with the standard TIL grammar
include "TIL.Grm"

% Add begin-end statements using grammar overrides
redefine statement
        ...                  % refers to all existing forms for [statement]
    |   [begin_statement]    % add alternative for our new form
end redefine

define begin_statement
    'begin                   [IN][NL]
        [statement*]         [EX]
    'end                     [NL]
end define

% No need to do anything except recognize the input, since the grammar
% includes the output formatting cues
function main
   match [program]
      _ [program]
end function

Example run:

<linux> cat begintest.til 
// Factor an input number
begin
var n; 
write "Input n please"; read n;
write "The factors of n are"; 
begin
var f; f := 2;
while n != 1 do 
    while (n / f) * f = n do
        // Got one - print it!
        write f; 
        n := n / f;
    end
    f := f + 1;
end end end
<linux> txl begintest.til TILbeginparser.Txl 
TXL v10.4a (15.6.05) (c)1988-2005 Queen's University at Kingston
Compiling TILbeginparser.Txl ... 
Parsing begintest.til ...
Transforming ...
begin
    var n;
    write "Input n please";
    read n;
    write "The factors of n are";
    begin
        var f;
        f := 2;
        while n != 1 do
            while (n / f) * f = n do
                write f;
                n := n / f;
            end
            f := f + 1;
        end
    end
end
<linux>